Physical characterization of diesel exhaust particles in exposure chambers

Environment International, Vol. 5, pp. 325-330, 1981 Printed in the USA. All rights reserved.

0160-4120/81/040325-06502.00/0 Copyright ©1982 Pergamon Press Ltd.

PHYSICAL CHARACTERIZATION OF DIESEL EXHAUST PARTICLES IN EXPOSURE CHAMBERS Sidney C. Soderholm BiomedicalScience Department,GeneralMotors ResearchLaboratories,Warren, Michigan48090, USA

Since the deposition of particulate in the respiratory system is strongly influenced by particle size, a correct assessment of this parameter is important for any inhalation experiment studying the potential health effects of air pollutants. Measuring the distribution of particles according to their aerodynamic diameter and mechanical mobility diameter is crucial in analyzing the deposition of submicron particles in the lower respiratory system. Cascade impactor measurements of diluted diesel exhaust in 12.6 m3 animal exposure chambers in the GMR Biomedical Science Department showed that the mass median aerodynamic diameter of the aerosol was 0.2 # m with 88% of the mass in particles smaller than I ~tm. Diffusion battery measurements showed that the mass median mechanical mobility diameter was about 0. II ~m. Transmission electron micrographs of particles deposited on chamber surfaces revealed both agglomerates and nearly spherical particles. The particles in these chambers are similar in size and shape to diesel particles described elsewhere. The flux of diesel particles to food surfaces was measured. Calculations of the expected daily dose by inhalation and by feeding showed that the "worst case" dose by feeding was only about one-tenth the dose by breathing.

tion experiment because lung deposition depends mainly on aerodynamic diameter for particles larger than about 0.3 ~tm and on mechanical mobility diameter for smaller particles. Knowing the distributions of particulate mass versus these two size parameters allows the deposited dose to be estimated directly. The calculation of deposited dose using any other size information requires extrapolations which have a high uncertainty, especially for irregularly-shaped particles such as those in diesel engine exhaust. We used a cascade impactor to measure the distribution of particulate mass versus aerodynamic diameter. Successively smaller particles are collected on each stage of an impactor as the air cascades through a series of jets. The construction of our impactors followed that of Mercer et al. (1970) but the number and size of the jets in each stage was changed. This allowed a smallest cutoff aerodynamic diameter of 0.23/~m while sampling at 4 L / m i n . The collected particles experienced a pressure reduction of less than 15 kPa. Commercially available impactors had too large a cutoff diameter causing most of the diesel particles to be collected on the backup filter, too low a flow rate requiring long sampling times, or too high a pressure reduction favoring evaporation of volatile components.

Introduction This paper covers three types of physical characterizations of the dilute diesel exhaust aerosol in the G M R Biomedical Science Department's 12.6 m3 animal exposure chambers. Such characterizations are necessary to define the exposure conditions. The three topics are (I) Health Effects Relevant Sizing, (II) Transmission Electron Microscopy, and (III) Particulate Doses by Feeding and Breathing.

I. Health Effects Relevant Sizing of the Diesel Aerosol in Exposure Chambers The objective of this work was to routinely measure the size distribution of the diesel exhaust aerosol in the G M R Biomedical Science Department's inhalation exposure chambers. This was necessary to assure the animals were given diesel exhaust particles of realistic size and to provide information relevant to deposited dose. There are many techniques available for obtaining size information. We chose two techniques which measure the distribution of particulate mass versus aerodynamic diameter and mechanical mobility diameter. These techniques are especially useful in an inhala325

326

Sidney C. Soderholm

The mass deposited on the glass collection plate in each stage and on the back-up filter was measured with an electrobalance. The raw data were analyzed assuming an ideal collection efficiency for each stage and plotting the data on log-probability graph paper, (Mercer, 1973). The cutoff diameter was calculated from information in the literature on the calibration of similar impactors (Newton et al., 1979). A typical result for the chambers containing diesel exhaust particles is shown in Fig. 1. The percent of mass on particles smaller than each cutoff diameter is plotted against the cutoff diameters on logarithmic probability paper. If the points fell on a straight line, the distribution would be log normal. The parameters reported for each distribution are (1) the mass median aerodynamic diameter (MMAD), and (2) the percent of mass on particles smaller than 1.0 ttm (PCI). The result of 54 measurements in all the chambers containing diesel exhaust was M M A D = 0 . 2 0 (--+0.03) #m and PC1=88 (+ 5)%. The uncertainties represent one standard deviation. Two points should be emphasized regarding this sizing technique: (1) slight revision of the reported size may be necessary after a full calibration of our impactors is completed, and (2) the multijet design had larger wall losses on the lower stages than expected (up to 20% of the total mass sampled), but correcting for this loss seems to change the mass median aerodynamic diameter by only about 10%. Because 50%-60% of the aerosol mass was caught on the backup filter of the impactor, it was necessary to SIZE DISTRIBUTION IN DIESEL EXPOSURE CHAMBER 10/18/78 10

6

-

size the aerosol by a different technique--one which is sensitive to smaller particles. The principle of the diffusion battery is that as air is pulled through a channel particles near a wall may hit it due to their Brownian motion and stick. The concentration of aerosol upstream and downstream of the channel was measured and the fraction penetrating the channel was related to the aerosol's distribution vs diffusion coefficient. The diffusion battery consists of several diffusion cells each of which is a matrix of channels. Some of the cells are equivalent to a single channel 4.6 km long. Figure 2 shows the data of 18 runs combined. The fraction of mass concentration penetrating the cell is plotted against the collection efficiency parameter of that cell. The collection efficiency parameter is a combination of the effective length of the cell and the volume flow rate of air passing through it. The line is the theoretical penetration curve for an aerosol of mass median mechanical mobility diameter=0.11 /~m. The width of the distribution is specified by og~4.5. This data analysis was done using the theory appropriate to channels of circular cross-section (Soderholm, 1979), whereas our channels were triangular. The difference should not be large. The results may have to be revised slightly once a full calibration is performed.

II. Transmission Electron Microscopy Many electron micrographs of diesel particles have been published (Frey and Corn, 1967; Vuk et al., 1976). Most show agglomerates roughly 0.1-0.3 ~m in size with some smaller singlets. Figure 3 shows particles which were collected from diluted engine exhaust using a TSI Model 3100 electrostatic sampler. Many chain and cluster agglomerates are seen. The pictures in Fig. 4 were taken from grids which had been attached to the chamber walls overnight. Note that many more small singlet particles are seen due to their preferential deposition by diffusion. Figure 5 shows some agglomerates and unusual particles which were

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10 30 50 70 90 Cumulative % of Mass Smaller Than the Stated Diameter

98

Fig. I. Typical log-probability plot of the size distribution of diesel particles in the exposure chambers as measured with a cascade impactor,

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,

5

~

,

,

,..

,

,

2

5

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Fig. 2. Analysis of diffusion battery data which is a summary of 18 samples from all three exposure chambers.

Physical characterization of diesel exhaust particles

327

Fig. 3. TEM micrographs of randomly chosen diesel exhaust particles collected from the tailpipe using an electrostatic sampler.

found in the chambers. The micrographs in Figs. 3 and 4 were taken at random on the specimen grids, but the micrographs in Fig. 5 were not. The conclusion drawn from these micrographs is that the diesel exhaust particles in our chambers are similar in size and shape to those previously reported.

III. Dose by Feeding and Breathing In a whole body exposure to diesel aerosols, the animals' intake of particulate occurs through feeding and grooming as well as through breathing. The dose received by all routes should be known to help understand possible causes of any biological effects which may be seen. The objective of this work was to estimate and compare the doses by feeding and breathing to the

animals in the G M R Biomedical Science Department's diesel exposure experiment. The dose by grooming was not estimated in this study. An experiment was performed to determine the mass flux of diesel particulate to food surfaces in this exposure. Transmission electron microscope specimen grids were placed on food surfaces in the exposure chamber overnight. The deposit was analyzed by transmission electron microscopy to obtain the volume surface density S v of deposited particles: S~ = 1.6× 10 - s cm3 particles cm 2 surface The equivalent volume diameter of each particle was estimated using a ruler. I estimated the uncertainty

328

Sidney C. Soderholm

Fig. 4. TEM micrographs of randomly chosen diesel exhaust particles collected by placing a specimen grid on the exposure chamber wall overnight.

in S~, to be about a factor of two due to the crude method available for estimating particle volumes of agglomerates from electron micrographs. The mass flux J of diesel particulate to food surfaces was calculated to be

be proportional to the mass concentration of airborne particles. The mass flux may be different for different size particles, feeder configurations, or air flows. The dose by feeding was estimated using the formula D F = JTA.

J = (1.4 × 10 -5 m / s ) C , where C is the mass concentration of airborne diesel particles. The following information was needed to calculate the mass flux: (1) the volume surface density of deposited particulate S v, (2) an estimate of the density of diesel particles (Op = 1.5 g/cm3), (3) the time H that the grid was exposed to diesel particles in the chamber, (4) the mass concentration during that exposure Cc~p, and (5) the fact that the mass flux should

The length of an exposure day T was 20 h. The area of food surface which the animals kept clean of particles was A. Estimating this area involved large uncertainty. The total area of food surface available for diesel particle deposition was estimated to be 30 cm 2 for the rats and 100 cm 2 for the guinea pigs. I used these values for A. This was equivalent to making the "worst case" assumption that the animals ate from all the available area of food keeping it clean of particles. This calcula-

Physical characterization of diesel exhaust particles

329

Fig. 5. TEM micrographs of selected diesel exhaust particles collected by placing a specimen grid on the exposure chamber wall overnight.

tion gave D F -- 3

D F =

~g m3 • C for the rats day mg

l0 7g ~ m3 . C for the guinea pigs. oay mg

If the animals ate from only one part of the food surface, the true dose by feeding would be much lower than this "worst case" estimate. The dose by breathing was estimated using the following formula: D B = Vm T C E .

The animal's minute volume Vm is the volume of air inhaled in a minute• The length of an exposure day T was 20 h, the mass concentration of airborne particles was C, and the deposition efficiency was E.

The available minute volume data (Guyton, 1947; McMahon et al., 1977; Crossfill and Widdecombe, 1961; Raabe et al., 1977) for rats and guinea pigs is shown in Fig. 6. Also shown are two correlations between body mass and minute volume based on data from a number of species. From the graph, I estimated the minute volume of the 400 g rats in this experiment to be 200-+50 cm 3 and that of the 1000 g guinea pigs to be 250-+ 100 cm 3. There was little data available for guinea pigs and it did not agree with the general correlation, so extrapolation to the minute volume of the 1000 g guinea pigs carried larger uncertainty than the minute volume estimation for the rats. The deposition efficiency E was taken to be 0.3 based on total deposition (head, stomach, lungs, etc.) seen by Raabe et al. [10] in experiments with rats and hamsters using an aerosol which was similar in size to diesel particles. The deposition efficiency was the same for the two rodent species, so I applied it to the guinea pigs as

330

Sidney C. Soderholm Table I. Relative daily dose.*

MINUTE VOLUME OF RATS AND GUINEA PIGS /

300

~

/

/

Rats Guinea Pigs

I j

Breathing

Eating

72 90

3 10

*Multiply by diesel particulate mass concentration in mg/m 3 to obtain dose in/tg/day. Minute Volume 100 (cm3)

dose by feeding is only about one-tenth of the dose by breathing for the animals in the GMR Biomedical Science Department's diesel exposure experiment. / / 30

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I 100

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300

1000

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Guyton(61 guinea pigs)

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Gu~on {35 rats)

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Crosfill el el {4 guinea pigsJ

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Crosfill el al {4 rats)

Raabe el al (30 male rats) + McMahon etal (Stats)

Raabe e, el. (28 ,sinai. . . . . )

- - V M = 4 . 1 9 M O 6 6 ( M c M a h o n etal.) ~ VM= 210 M 0 75 (Guyton)

Fig. 6. Minute volume of rats and guinea pigs from the literature.

well as to the rats. This calculation gave D B = 72 /~g m 3 • C for the rats daymg D B = 90 ~g m3 • C for the guinea pigs. aay mg The comparison of the doses by feeding and breathing in Table 1 shows that the "worst case" estimate of the

References Crosfill, M. L. and Widdecombe, J. G. (1961) Physical characteristics of the chest and lungs and the work of breathing in different mammalian species, J. Physiol. ( London ) 158, 1- 14. Frey, J. W. and Corn, M. (1967) Physical and chemical characteristics of particulates in a diesel exhaust, Am. Ind. Hyg. Assoc'. J. 2,8, 468-478. Guyton, A. C. (1947) Measurement of the respiratory volumes of laboratory animals, Am. J. Physiol. 150, 70-77. McMahon, T. A., Brain, J. D., and Lemott, S. (1977) Species differences in aerosol deposition, in Inhaled Particles IV. Pergamon Press, Oxford, England. Mercer, T. T. (1973) Aerosol Technology in Hazard Evaluation. Academic Press, New York. Mercer, T. T., Tillery, M. I., and Newton, G. J. (1970) A multi-stage low flow rate cascade impactor, Aerosol Sei. 1, 9-15. Newton, G. J., Raabe, O. G., and Mokler, B. V. (1979) Cascade impactor design and performance, Aerosol Sci. 10, 163-175. Raabe, O. G., Yeh, H. C., Newton, G. J., Phalen, R. F., and Velasquez, D. J. (1977) Deposition of inhaled monodisperse aerosols in small rodents, in Inhaled Particles IV, pp. 3-21. Pergamon Press, Oxford, England. Soderholm, S. C. (1979) Analysis of diffusion battery data, Aerosol Sci. 10, 163-175. Vuk, C. T., Jones, M. A., and Johnson, J. H. (1976) The measurement and analysis of the physical character of diesel particulate emissions. Paper 760131, Society of Automotive Engineers Transactions.